1. Field of the Invention
The present invention relates to an image processing apparatus that can handle feature information specific to a paper sheet of an individual paper document on which letters are printed (hereinafter, such feature information also referred to as a dactylogram of paper or a paper dactylogram), an image processing method, a program for the image processing method and its storage medium.
2. Description of the Related Art
In view of the rapid progress of document computerization in recent years, a technique is desired to appropriately determine the authenticity (true or false) of a paper document in order to ascertain that various types of documents issued by outputting on paper are original paper or are authenticated.
Generally, as means for preventing counterfeiting of documentation, there is known a method of printing an identification code on the documentation itself and, on this occasion, using an advanced printing technique to make counterfeiting difficult or using unavailable special ink to print the identification code. A method is also known of bonding to documentation a counterfeiting prevention sheet formed by a special technique such as holography. The method of using special paper or a special printing technique has a problem of inconvenience and high cost due to using a printing medium which is not widely-distributed. Nowadays, there is proposed a technique of using non-reproducible random patterns on a paper surface for identification (for example, see Japanese Patent Laid-Open No. 2004-102562, hereinafter referred to as the reference 1). According to the technique disclosed in the reference 1, the unique state information produced by a paper manufacturing process, for example, a fiber pattern of fibers forming the paper or a lack of uniformity of thickness, is scanned and the scanned image data is stored in advance, then the state information is compared to state information obtained from the same place on a tested paper. The degree of matching is calculated in the comparison, enabling confirming of whether or not the tested paper is the original paper.
The dactylogram of paper disclosed in the reference 1 is very susceptible to folds or wrinkles of a printing medium. Therefore, the reference 1 discloses as follows in its paragraph 0094: “Paper would be susceptible to fold or wrinkle due to secular changes, state of preservation or a usage state. Because of this, an observation region of tested paper registered by the above registration processing would not be possibly read out in the same manner at the registration. To address such a case, means for specifying observation regions over a plurality of places on the tested paper is valid. Particularly, for example as shown in FIG. 17, five places is selected in a particular range (a rectangular region as shown in the drawing herein) including points Q1 to Q5 on a paper surface of tested paper 414, the five places are specified as observation regions S1 to S5. In this case, robustness is improved by employing an approach of giving priorities to the respective observation regions S1 to S5 and to compare/determine them in order as necessary.”
That is, the reference describes that comparison/determining of a sheet is more precise by registering a plurality of paper dactylograms rather than registering a single paper dactylogram. In the reference, a paper dactylogram is image information, the image representing a pattern of paper fiber specific to a paper sheet of a paper document.
Particularly, if paper is not stained at all or wrinkled, a dactylogram of the paper does not change at all. Therefore, a previously registered dactylogram of the paper matches a scanned dactylogram of paper on a sheet (or the degree of similarity between them is extremely high).
Otherwise, (1) if the paper itself is replaced, or (2) if folds or wrinkles of the paper cause the dactylogram of the paper to change, the previously registered dactylogram of the paper does not match the scanned dactylogram of paper on the sheet.
In the above case, (1) if the paper itself has been replaced, then the degree of similarity is measured to be very low (for example, the kind of paper itself differs from the original paper), or the degree of similarity is measured to be a value which is not very high but not very low (for example, both fiber patterns are accidentally similar to each other).
Also, in the above case, (2) if folds or wrinkles of paper cause a change in the dactylogram of the paper, the degree of similarity is measured at value which is not very high but not very low.
That is, if the degree of similarity between previously registered image information and a scanned image information on a sheet indicates a value which is not very high but not very low, this means it cannot be determined whether the paper itself has been replaced, or the paper itself has not been replaced (but has suffered from folds or wrinkles).
Therefore, when the degree of similarity is not very high but not very low, as for guaranteeing that the paper is the original paper, there is possibility of a mistake. On the other hand, the possibility of a mistake lies in negating that the paper is the original paper. In other word, there is possibility to cause a wrong determination result.